This Program Project explores theoretical and practical aspects of clinical decision-making, focused on the decision by a physician to order a diagnostic test or procedure for evaluation of a diagnostic hypothesis. A multidisciplinary research and developmnt team has been assembled to pursue four aspects of this problem: (1) To develop clinical decision support programs to aid the referring physician in the quantitative assessment of probabilities implicit in the decision to order a test, and seek to demonstrate a beneficial effect on the utilization of tests; (2) to develop data bases characterizing diagnostic efficacy and yield of those tests for which consultations are considered likely to be beneficial; (3) to implement an artificial intelligence model of decision-making, which uses the concept of threshold for diagnosis to link probabilities and actions; and (4) to investigate theoretical aspects of the concept of threshold, aimed at characterizing the factors involved.